(x+1/x)^2=x^2+2*x*1/x+1/x^2=2+x^2+1/x^2=3 所以x^2+1/x^2=1x^3+1/x^3=(x+1/x)(x^2-x*1/x+1/x^2)=3*0=0
x+1/x=3平方x^2+2+1/x^2=9x^2+1/x^2=7x^3+1/x^3=(x+1/x)(x^2-1+1/x^2)=3*(7-1)=18
